scripts: zephyr_flash_debug: flash like pyocd.sh

Add support for flashing targets compatible with pyocd.sh.

Tested on 96b_nitrogen, nrf52_blenano2, and frdm_k64f, with and
without PYOCD_BOARD_ID. Additionally, frdm_k64f was tested with
PYOCD_DAPARG_ARG='limit_packets=True'.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>